1. Field of the Invention
The present invention relates to vapor phase corrosion inhibition and to vapor phase transport of corrosion inhibitors, and more particularly to inhibition of corrosion due to carbon dioxide and hydrogen sulfide in gas lift processes.
2. Description of the Prior Art
Gas lift is a technique applied to oil wells to produce fluids from the wells after the natural flow from the well ceases or to supplement the natural flow. According to this technique, when the pressure in the well in insufficient to eject subterraneous fluids up through the well for collection at an acceptable rate, gas from a subterraneous formation is collected and injected at high pressure down the annulus of the well to displace the fluid from the point of gas injection to the surface. Typically such gas is methane, but other gases may also be present in minor amounts and exacerbate corrosion problems in the gas lift system and well. Moreover, the produced liquids themselves may contribute CO.sub.2 and H.sub.2 S to the system.
Each well may be equipped with a pump for compressing the gas and injecting it into the well. However, a large number of wells often populate a localized area of several square miles and it is economically advantageous to use a single pump to service all such wells. For example, 500 wells over 40 platforms might populate an offshore producing area. Thus, instead of employing 500 pumps (or 40 pumps), a single pump can be employed with conduits communicating between the pump and each of the 500 wells.
An intractable problem associated with this technique, however, has been the carbon dioxide and/or hydrogen sulfide corrosion of metal surfaces in the system, including surfaces of numerous conduits that carry production fluids and metal surfaces within the well. Such problems include the difficulty in maintaining a corrosion inhibitor in a vapor phase, particularly through the lengths of conduits to the wells. As a result, corrosion inhibitors have been added to the lift gas at the well sites, thereby requiring an injection pump at each well site for the injection of the inhibitor at that site. Accordingly, 500 relatively localized wells would require 500 pumps.
Another problem is that conventional corrosion inhibitors, regardless of their effectiveness in other applications, have not been found to be sufficiently effective in inhibiting corrosion in the vapor phase. Thus, for example, although U.S. Pat. No. 4,555,576 to Oude Alink, a co-inventor herein, notes in passing that the compositions of that patent may be useful as corrosion inhibitors, it contains no suggestions that such compositions would be useful as vapor phase inhibitors. In fact, most such compositions as broadly defined in U.S. Pat. No. 4,555,576 have a molecular weight too high to permit an effective vapor phase concentration. For example, the only such dihydrothiazole known to the inventors to be commercially employed as a component in an inhibitor package is 2,5-dihydro-2,2-pentamethylene-5,6-tetramethylene thiazole, which has a boiling point too high to be a practical vapor phase inhibitor. Generally, however, corrosion inhibitors have been found to be of insufficient effectiveness in vapor phase applications, and typically must be employed in combination with a neutralizer, usually an amine.
Moreover, it is recognized that in order for an inhibitor to inhibit corrosion of a metal surface, the inhibitor must show an affinity for that surface, thereby to provide effective inhibition. Conventional inhibitors have been adapted to exhibit greater affinity to metal surfaces by increasing the size of the inhibitor. In the case of a vapor phase inhibitor, such techniques are self-defeating because it converts the inhibitor to a liquid phase.
Accordingly, the industry is still searching for effective vapor phase or volatile corrosion inhibitors that may be employed in gas lift technology not only to afford corrosion protection to surfaces contacted by the vapors, but also to provide equally distributed corrosion inhibitor into the well-produced fluids. A vapor phase inhibitor is sought therefore, that would provide equal distribution from an injection point or a plurality of injection points to inhibit corrosion in the produced liquids (which contain, for example, carbon dioxide or hydrogen sulfide). Such distribution has not been found to result from the application of liquid inhibitors through manifolds.